Today's cellular communication systems provide not only voice services, but also mobile broadband services all over the world. As the number of applications for cell phones and other wireless devices continues to increase, and consume higher ever-increasing amounts of data, an enormous demand for mobile broadband data services is generated. This requires telecom operators to improve data throughput and maximize the efficient utilization of limited resources.
As the spectrum efficiency for the point-to-point link already approaches its theoretical limit, one way to increase data throughput is to split big cells into smaller and smaller cells. When the cells become closer to each other, however, interferences among adjacent cells become more severe, and the cell splitting gain saturates. Furthermore, it is becoming more difficult to acquire new sites to install base stations for the operators and the costs are also increasing. Therefore, cell-splitting alone cannot fulfill the demands.
Recently a new type of network deployment called Heterogeneous Network (HetNet) has been proposed and is attracting a lot of interest and effort by the industry. In HetNet, another tier of multiple low-power nodes (LPNs) is added as cells in the existing macro base station's coverage areas. The low power nodes may operate on the same carrier frequency as the macro node or on different carrier frequencies. For efficient network operations, it is desirable that the low power nodes/cells can be discovered by the user equipment (UE). The legacy solution to discover cells (e.g., LTE R8-R11) is to let the LTE base stations (eNBs) to transmit the primary synchronization signals (PSS) and secondary synchronization signals (SSS) every 5 ms. The UE searches for these PSS/SSS signals when it is looking for new cells. During the evolution of the 3rd Generation Partnership Project (3GPP) standard it has been found that this legacy approach is not sufficient to deal with at least the following new challenges coming from the HetNet deployment:                a) The mutual interference between different PSS/SSS sequences from different cells is quite high. The PSS/SSS are not designed for dense deployment of cells as it now occurs in HetNets. Consequently, when many cells are present, the UE is only able to find a few of them.        b) In order to save power and to reduce interference, it is desirable to switch a small cell into an idle mode when the small cell is not needed (e.g. when it has low or no load). During the idle period, it would be desirable to also switch off the PSS/SSS transmission. However, the UE will be unable to discover the presence of the cell when PSS/SSS is shut-off.        